This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. Primary support for the subproject and the subproject's principal investigator may have been provided by other sources, including other NIH sources. The Total Cost listed for the subproject likely represents the estimated amount of Center infrastructure utilized by the subproject, not direct funding provided by the NCRR grant to the subproject or subproject staff. The aim of the current project is the synthesis and characterization of high-valent Fe-oxo complexes and their application as catalysts for selective C-H bond activation. Our strategy for stabilizing high-valent diiron units involves increasing the electron density at the iron ions by using strongly electron-donating ligands. For the ligands of the first generation, these involved strongly electron-donating s- and p-donors, namely phenolates. Detailed investigations of these complexes, particularly by -ray absorption spectroscopy (XAS), revealed that 1) the oxidation is ligand-centered and not metal-centered and 2) the oxidized species dissociate into mononuclear species. We used these results for the optimization of the ligand design. The new dinucleating ligands of the second generation have been synthesized and first investigations on their FeIIFeII, FeIIFeIII, and FeIIIFeIII complexes demonstrate the suitability of these complexes for generating high-valent dinuclear complexes, which should result in metal-centered oxidations and should possess higher stability as dinuclear entities. We propose the use of XAS to investigate the molecular and electronic structures of the parent complexes in solution as well as of the one- and two-electron oxidized species.